It is established medical practice to introduce a flexible tubular catheter into a desired position in a patient's body by means of a catheter placement unit which includes an introducing needle extending externally or internally of the catheter and projecting beyond the end of the catheter. In use of such a catheter placement unit to place a catheter within a patient's vein for supply of infusion fluid, for example, the needle tip projecting beyond the end of the catheter serves to puncture the patient's skin and to locate the catheter within the vein. When the catheter is suitably positioned, as may be determined by the flow of blood in the hub of the needle, the needle may be withdrawn leaving the catheter in position in the vein. Typically the needle is completely withdrawn from the catheter and safely disposed of prior to the connection of a fluid line to the catheter so as to prevent damage to the patient by the needle during infusion. However the need to withdraw the needle from the catheter leads to a risk that doctors or nurses will accidentally prick themselves with the needle, and this can be highly dangerous due to the risk of transfer of blood-related diseases.
U.S. Pat. No. 3,463,152 and U.S. Pat. No. 3,536,073 disclose catheter placement units of a type in which the needle is withdrawn into a housing connected to the catheter and locked therein after placement of the catheter in the patient's body, in order to prevent damage to the patient from the needle. However such catheter placement units are complex and bulky in that they require a needle housing which remains attached to the catheter in use, and are not suitable for use in applications where the needle is to be withdrawn completely from the catheter and disposed of.
U.S. Pat. No. 4,978,344 discloses a catheter placement unit of a type in which the needle is withdrawn completely from the catheter and disposed of after placement of the catheter in the patient's body, and in which the action of withdrawal of the needle from the catheter causes a tethered cap portion to be drawn over the needle tip. However the drawing of the cap portion over the needle tip relies on frictional engagement of the cap portion by the catheter hub, and it is possible to withdraw the needle from the catheter without triggering the required capping action. Also, because of the form of the capping portion, it is possible for the needle tip to be re-exposed after capping.